Debian

Since Debian doesn't see updates inbetween releases unless there's some urgent reason for them, the software in the distribution's repositories ages. The sabnzbdplus package is no exception. If you want a newer version in Debian squeeze (6.0) or wheezy (7.0), these are the options:

1) Use the ppa. Although designed for Ubuntu, community members found it works with Debian too ;D. Updates come in automatically, so you'll be the first (along with some 10,000+ Ubuntu users) to enjoin the latest bugs features. The instructions below closely follow those for Ubuntu, see there for context. All commands need root access, use su or sudo as appropriate or execute them from a root shell.

2) Rebuild the source package from testing on a squeeze system. Needs manual work for every update, may not work (or not without extra work) with future updates entering testing, but it does at the time of writing. Instructions straight from the Debian wiki are here. Nice of you like to dig in. Scary for noobs.

3) Run from source. Download the source release from sourceforge, extract anywhere you like, and just run SABnzbd.py. Dependencies should have been taken care of if you had the (old) sabnzbdplus package installed, otherwise look at that for guidance. Updating is as simple as overwriting the old program files with a newer release and running is simple enough too, but you'll have to take care of things like changes in dependencies and a proper init script yourself.

Some notes:

No warranty. For anything, in this post or linked from here.
For major updates, better finish any existing download queue first and backup your settings.
Whatever you do, don't try using the binary deb files directly from testing or unstable: they will not work without the rebuilding.
For even older Debian releases (yeah, that's you, lenny addicts), there's an install script by user bluechip in this topic.